A solid battery is known as a battery excellent in safety. The solid battery includes a cathode, an anode, and a solid electrolyte layer as a separator layer between the cathode and anode. The cathode mixture composing the cathode of a solid battery includes a cathode active material, an electroconductive material, a solid electrolyte, and the like. Specifically, the cathode can improve its electron conductivity by including an electroconductive material in the cathode mixture. As the electroconductive material, particulate electroconductive materials, fibrous electroconductive materials and the like are known (WO 2013/146454), and various studies have been done regarding compounding forms of electroconductive materials in the cathode mixture.
On the other hand, though not being a technique according to a solid battery, JP 2007-080652 A for example describes that it is possible to obtain a lithium ion battery having a high discharging capacity at a high speed charge rate and exerting a performance of sufficient charge-discharge rate by using an electroconductive material consisting of two or more kinds of particles whose sizes or forms are different from each other, to compose a cathode of the lithium ion battery. JP 2008-034376 A describes that it is possible to obtain a lithium ion battery having an excellent high speed discharging performance by using a cathode mixture containing a particulate electroconductive material whose primary particle size is 10 to 100 nm, a fibrous electroconductive material whose fiber size is 1 nm to 1 μm, and an electroconductive material whose aspect ratio is 2 to 50, to compose a cathode of the lithium ion battery. JP 2002-063937 A describes that it is possible to obtain a lithium ion secondary battery having a largely improved low temperature characteristic and rate characteristic by using a cathode mixture including a particulate electrocondutive material whose particle size is 3 μm or more and a particulate electroconductive material whose particle size is 2 μm or less or a fibrous electroconductive material whose aspect ratio is 3 or more and whose fiber size is 2 μm or less, to compose a cathode of the lithium ion secondary battery. In addition, though not being a technique according to a solid battery but an anode, a technique to use two kinds of graphite electroconductive materials whose average lengths are different from each other is also known.